1. Field of the Invention
The present invention relates to a synthetic lubricating oil composition having good oxidative stability or oxidation resistance. The present invention also relates to a process for preparing such a composition.
2. Description of the Prior Art
Lubricating oils which are recycled in use should generally stand long-term use. For this purpose, it is usual to employ mineral lubricating oils wherein highly refined mineral oils are formulated with appropriate antioxidants. However, the mineral lubricating oils undesirably have a limitation with respect to the oxidative stability. This makes it difficult to subject the mineral lubricating oil to long-term use such as in machines whose portions to be lubricated are exposed to severe temperature conditions.
Accordingly, synthetic ester lubricating oils such as diesters, polyol esters and the like have been developed and commercialized as having better oxidative stability and have now been in general use. Although these synthetic ester lubricating oils have good oxidative stability, inherent disadvantages are involved in that they suffer hydrolysis to produce acids, cause sealing agents to be swollen and are expensive. Accordingly, limitation is placed on their range of utility.
Hydrogenated products of poly-.alpha.-olefins have been widely known as a synthetic lubricating oil having high oxidative stability. The oxidative stability is better than those of mineral oils but is not so high as those of synthetic ester lubricating oils.
Recently, attention has been drawn to high oxidative stability of naphthalene derivatives. There are provided lubricating oil compositions which are obtained by mixing alkylnaphthalenes used as a base oil for the lubricating oil and specific types of compounds at a defined ratio. These are set forth, for example, in Japanese Laid-open Patent Application Nos. 59-147096 and 59-147097. These compositions are not still satisfactory when applied to the fields where high oxidative stability is essentially required.